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DOI: 10.1039/D4NJ02455A (Paper) New J. Chem., 2024, Advance Article

Construction of an off–on fluorescence sensing platform for FRET-based detection of coumarin in cosmetic samples

Ruo-Qian Xua, Cai Shia, Xue-Mei Donga, Xuan Xiaoa and Yu-Jie Ding*ab
aSchool of Chemical and Environmental Engineering, Anhui Polytechnic University, Wuhu, Anhui 241000, P. R. China. E-mail: dyj@ahpu.edu.cn
bAnhui Provincial Key Laboratory of Discipline Co-construction on Intelligent Equipment Quality and Reliability, Wuhu, China

Received 28th May 2024 , Accepted 21st August 2024

First published on 22nd August 2024

Abstract

2-Hydroxypropyl-β-cyclodextrin (2-Hp-β-CD)-functionalized CdTe quantum dots (QDs) were used for coumarin-sensing with high sensitivity and selectivity. This fluorescence resonance energy transfer (FRET)-based sensor was established by attaching rhodamine 6G (Rh6G) to the QDs via a 2-Hp-β-CD linker. Rh6G could get into the cavity of 2-Hp-β-CD, and then trigger the FRET between the QDs and Rh6G, providing an “off” state of the QDs. However, the effect of FRET was weakened by the coumarin, because of its larger hydrophobic association constant with 2-Hp-β-CD, resulting in the dissociation of Rh6G. In the presence of additive coumarin, the fluorescence intensity of QDs@2-Hp-β-CD@Rh6G increases (an “on” state). The FRET-based off–on nanoprobe shows a good linear relationship in the range of 0–25 μM with a good correlation coefficient (R2 = 0.9986). We also obtained an excellent result using this spectrophotometric method for coumarin sensing in cosmetics, which has potential for application in the biomedical area. We expect this sensing platform could be useful in other analogous nanoprobes in relevant fields.

Introduction

Coumarin is a kind of phytochemical that belongs to the naturally occurring polyphenolics and is distributed widely within the plant kingdom.1 As an important spice ingredient, it is commonly used as an odorant and a deodorizer to formulate perfumes and cosmetics.2 However, because of its potential carcinogenicity to the human body, its concentration in these products should be strictly limited.3 It is of great importance to detect coumarin, and many methods have been developed, such as high-performance liquid chromatography (HPLC),4 gas chromatography,5 and so on. These techniques in most cases require large amounts of hazardous organic solvents and tedious pretreatments.6 At present, fluorometry is of great significance due to its high sensitivity, quick response, and good portability. A lot of fluorescent substances have been used for sensing, such as metal–organic frameworks,7 quantum dots (QDs),8,9 carbon dots,10,11 metal nanoclusters,12 and so on.

As a class of inorganic nanomaterials, QDs have been developed for application in energy conversion, imaging, drug delivery, etc., on account of their exceptional optical performance.13–15 Especially, as fluorescence sensors, QDs have the advantages of rapid reaction and simple operation, as well as biocompatibility.16 QD fluorescent composites, a kind of assembly materials different from pure QDs, not only have the characteristics of traditional compounds but also possess the fluorescence properties of QDs. In this kind of composite, the individual parts still maintain their characteristics, and can also bring about synergistic effects, improving the practicability.17–19 At present, QD composites have been used in drug carriers,20 imaging,21 solar cells,22 and sensors.23–25

Fluorescence resonance energy transfer, denoted as FRET, is a radiationless process, in which the energy of the excited fluorescent donor is transferred to the acceptor, resulting in the quenching of fluorescence. The FRET mechanism has been widely used in the analysis of various substances.26–28 To achieve excellent FRET efficiency, it is important to get a better connection type to bring the acceptor into sufficient proximity with the donor, through host–guest interactions,29 layer-by-layer assembly,30 electrostatic adsorption,31 and so on.

So far, the FRET from QD-based fluorescent donors to recognition units has been largely investigated for developing new sensing methods.32 Feng33 designed a g-CNQDs@Zn-MOF sensor for the detection of riboflavin. Employing CdS QDs as donors, Ganiga34 could detect vitamin C by the FRET effect. Zhu35 prepared a FRET probe through cysteamine-stabilized CdTe QDs and sodium citrate-stabilized Au nanoparticles for the detection of glutathione in real human plasma samples. Zhang36 synthesized Zn-doped CdTe QDs embedded into silica nanoparticles, which served as a FRET donor conjugated with an acceptor, and constructed a FRET-based sensor for the detection of Hg2+. In addition, some other optical chemosensors using QD-based assembly as a fluorophore have been developed for the sensing of thrombin,8 positive ions,37 tetracycline,38 insulin,39 and so on.

Herein, in this work, we design a FRET-based nanoplatform to be used for the direct assay of coumarin (Scheme 1). The CdTe QDs as a donor having a strong fluorescence emission peak at 554 nm are linked by 2-hydroxypropyl-β-cyclodextrin (2-Hp-β-CD). Rhodamine 6G (Rh6G) possesses a wide UV-vis absorption spectrum centred at 525 nm, and is used as an energy receptor, namely a recognition moiety. It can be crammed into the cavity of the 2-Hp-β-CDs through the interaction of host–guest, which can cut down the distance between them. Because of a good overlap between the donor and the receptor, the FRET effect can be triggered and the energy is transferred from QDs to Rh6G, resulting in the fluorescence quenching of the donor (an “off” state). When coumarin joins, it replaces Rh6G because its binding coefficient is larger than that of Rh6G to 2-Hp-β-CD. The FRET is blocked off and the fluorescence recovered (an “on” state). Therefore, an off–on biosensing nanoplatform for direct detection of coumarin is constructed based on the FRET effect in the QDs–rhodamine 6G assembly.


image file: d4nj02455a-s1.tif
Scheme 1 A schematic illustration of the assembly process for the coumarin sensor.

Experimental

Reagents and instruments

The major reagents such as Te (99.9%), NaBH4 (99%), CdCl2·2.5H2O (99%), mercaptoacetic acid (TGA, 95%), and 2-hydroxypropyl-β-cyclodextrin (denoted as 2-Hp-β-CD, 98%), and rhodamine 6G (Rh6G, 95%) and coumarin were purchased from Tansoole reagent platform and used directly. All the other reagents are used directly with purification.

An XRD-6000 diffractometer (Bruker D8 Advance) was used to obtain X-ray diffraction (XRD) data, at a scanning rate of 0.05 degrees per s and in a 2θ angle range of 0–80 degrees. Transmission electron microscopy (TEM) images were obtained using a transmission electron microscope with an acceleration voltage of 200 kV provided by American FEI Company. IR spectral tests were performed with a Fourier transform IR spectrometer from Nicolet iS20 FT-IR Spectroscopy (Thermo Scientific). The LS5 UV-vis analyzer from Shanghai Jingke Company was used to get the UV-vis absorption spectra. Fluorescence spectra were achieved by using a F-7100 fluorescence spectrophotometer (HITACHI). The time-resolved fluorescence decay was investigated using a high-resolution FLS1000 fluorometer of Edinburgh Instrument, by using a 360 nm laser.

Material preparation

Preparation of CdTe QDs. 51.0 mg (0.4 mmol) of Te powder and 37.8 mg (1.0 mmol) of NaBH4 were both put into a small beaker and then rapidly pipetted into a three-neck flask with 20 mL of ultrapure water in portions. The solution was reacted at 60 °C for 30 min under N2 protection to obtain a pale purple NaHTe solution.40

114.2 mg (0.5 mmol) of CdCl2·2.5H2O was mixed with 242 μL of TGA in 100 mL of ultrapure water. 1.0 M NaOH was added dropwise under magnetic stirring until the pH became greater than 10. As the pH increased, the solution gradually became clearer. It was transferred into a three-neck flask and refluxed at 80 °C in an N2 atmosphere for 30 min. 5 mL of aforementioned NaHTe was quickly injected into the above-mixed solution, and continued to be refluxed for 40 min to obtain water-soluble CdTe QDs, which were centrifuged and washed with ethanol.

Design of CdTe@2-Hp-β-CD@Rh6G

1 g of 2-Hp-β-CD in 60 mL water was put into the QDs under reflux for 2 h at 100 °C to achieve CdTe@2-Hp-β-CD, and centrifuged to get the intermediate product.41 CdTe@2-Hp-β-CD was added with Rh6G and reacted for 24 h, after which the desired composites were obtained. Free Rh6G was wiped off by centrifugation and the product was purified by ethanol.

Coumarin sensing and the selectivity test

5 mg CdTe@2-Hp-β-CD@Rh6G in 10 mL aqueous solution was prepared. 100 μL of this solution and various concentrations of coumarin were mixed. Their fluorescence spectra were obtained at 400 and 700 nm wavelengths. The selectivity of CdTe@2-Hp-β-CD@Rh6G was determined by mixing with the interference substances, respectively, and their fluorescence spectra were determined.

Sensing of coumarin in biological medicine samples

The coumarin concentrations in all kinds of cosmetics were detected. 1 g of the sample (sunblock, mist spray, and emulsion) and 10 mL of methanol were both placed in a centrifuge tube under ultrasound conditions. After 30 minutes, 10 mL water was added and centrifuged. The upper solution after centrifugation (1 mL) was mixed with CdTe@2-Hp-β-CD@Rh6G to detect the fluorescence spectrum.

Results and discussion

Characterization of the relevant materials

Fig. 1a and b clearly show that the CdTe QDs have good dispersity and the particles are spherical with a diameter of 3.2 nm. Fig. 1c and d show the TEM images of CdTe@2-Hp-β-CD@Rh6G assembled materials. Their corresponding particle size distribution histogram is shown in Fig. 1e. As observed from these images, the composites have an average diameter of about 5.8 nm with a relatively uniform distribution. Because of the modification, although organic compounds are not visible from TEM images, the assembled materials still possess good dispersion with a larger size. At the same time, the XRD pattern of the pure QDs is shown in Fig. 1f, which has obvious peaks at 23.8°, 39.8° and 46.4°. These peaks can be assigned to the (1 1 1), (2 2 0), and (3 1 1) planes, which belong to the cubic zinc blende structure of pure CdTe (JCPDS 15-0770). These results indicate that the CdTe QDs have been successfully synthesized as expected.
image file: d4nj02455a-f1.tif
Fig. 1 TEM images and particle size distribution histograms of CdTe (a) and (b) and CdTe@2-Hp-β-CD@Rh6G (c)–(e); XRD characterization of CdTe QDs (the inferior graph shows the data of pure CdTe (JCPDS 15-0770)) (f).

The assembling processes were confirmed from the FT-IR spectra (Fig. 2). The CdTe QDs have two peaks at 1386 and 1581 cm−1 which are from the TGA molecule.42 The absorption peak at 1023 cm−1 corresponds to the C–O/C–C stretching vibrations and O–H bending vibrations of 2-Hp-β-CD, which can also be clearly seen for CdTe@2-Hp-β-CD. At the same time, the peaks at 2950 and 1385 cm−1 are characteristic peaks of the methyl groups from 2-Hp-β-CD, which can also be observed for CdTe@2-Hp-β-CD. In addition, new absorption peaks appeared at 1240 and 1100 cm−1 for the cyclodextrin-modified QD materials, corresponding to the asymmetric stretching vibration of C–O–C from esters. Therefore, it could be further confirmed that 2-Hp-β-CD has been linked to the surface of CdTe QDs. In Fig. 2b, the absorption peaks of Rh6G at 1367 and 1637cm−1 correspond to the stretching vibrations of C–N and C[double bond, length as m-dash]O, respectively, and CdTe@2-Hp-β-CD@Rh6G also showed absorption peaks at 1367 and 1637cm−1. These results show that the CdTe@2-Hp-β-CD@Rh6G nanoprobe has been synthesized.


image file: d4nj02455a-f2.tif
Fig. 2 FI-IR spectra of 2-Hp-β-CD (red), CdTe QDs(black), CdTe@2-Hp-β-CD (blue) (a) and Rh6G (red), and CdTe@2-Hp-β-CD @ Rh6G (blue) (b).

Detection of coumarin

Evaluation of the FRET mechanism. Absorption peaks of CdTe QDs, CdTe@2-Hp-β-CD, and Rh6G in the range of 400–650 nm are presented in Fig. 3a(a), (b), and (g). A sharp absorption peak was observed for Rh6G while the other two components have no absorption peak at 525 nm. In the meantime, the fluorescent spectra of CdTe QDs, CdTe@2-Hp-β-CD, CdTe@2-Hp-β-CD@Rh6G, and CdTe@2-Hp-β-CD@Rh6G added with 10 μM coumarin were recorded using a 360 nm excitation light source (Fig. 3a(c, d, e and f)). The pristine QDs have strong emission at 554 nm, however, their fluorescence intensity decreases after assembling with 2-Hp-β-CD, which can be justified perhaps as the emission of QDs can be inhibited by the other materials in the composite.43 Sequential reduction of emission intensity occurs after further recombination with Rh6G because of FRET. This is because of the overlap between the QD emission spectrum (donor) and the Rh6G absorbance spectrum (receptor).44 Therefore, the FRET effect takes place, leading to the fluorescence quenching of QDs (“off” state). Nevertheless, a noticeable recovery of the fluorescence intensity occurs after the addition of coumarin (“on” state). The association constant of coumarin with 2-Hp-β-CD is greater than that of Rh6G with 2-Hp-β-CD, therefore, coumarin can replace Rh6G to be loaded into the cavity of 2-Hp-β-CD.29 Coumarin is not an acceptor, hence, the FRET is blocked and the QD fluorescence recovers. Based on the above analysis, it can be seen that this system can realize the sensing of coumarin.
image file: d4nj02455a-f3.tif
Fig. 3 (a) The absorption spectra of 0.5 mg mL−1 CdTe QD solution (a), CdTe@2-Hp-β-CD (b) and Rh6G (g); fluorescence spectra of 0.5 mg mL−1 solution of CdTe QDs (c), CdTe@2-Hp-β-CD (d), CdTe@2-Hp-β-CD@Rh6G (e) and CdTe@2-Hp-β-CD@Rh6G + 10 μM coumarin (f) obtained under 360 nm light; (b) fluorescence decay spectra of CdTe@2-Hp-β-CD and CdTe@2-Hp-β-CD@Rh6G at 554 nm emission wavelength obtained with a 360 nm source.

To further verify the FRET behavior between QDs and Rh6G, the fluorescence lifetimes of the CdTe@2-Hp-β-CD and CdTe@2-Hp-β-CD@Rh6G were recorded at 554 nm with a 360 nm source. As indicated in Fig. 3b, CdTe@2-Hp-β-CD displays three exponential decay processes with the greatest lifetime of 28.9 ns. The lifetime of CdTe@2-Hp-β-CD@Rh6G would reduce to 12.4 ns with two exponential decay processes, further demonstrating a FRET effect from QDs to Rh6G.45

Optimization of material preparation and sensing time

The amount of 2-Hp-β-CD is an important factor for the FRET effect because only Rh6G that was loaded into the cavity of 2-Hp-β-CD could cause FRET. In the synthesis of CdTe@2-Hp-β-CD@Rh6G, different amounts of 2-Hp-β-CD were added to get various CdTe@2-Hp-β-CD@Rh6G composites. Their relative quenching degree (denoted as (I0–I)/I0) at 554 nm was measured (Fig. 4a). We can see that the quenching degree increases when the 2-Hp-β-CD amount increases until 1.0 g. Because the increased 2-Hp-β-CD could offer more space to accept more Rh6G, the FRET could become much stronger. When the amount of 2-Hp-β-CD is greater than 1.0 g, the intensity has little variation. Therefore, 1.0 g of 2-Hp-β-CD was chosen for further experiments.
image file: d4nj02455a-f4.tif
Fig. 4 (a) The effect of the amount of 2-Hp-β-CD on the relative fluorescence quenching degree of the composites (I0 is the original intensity of CdTe, I is the intensity of the materials with the corresponding amount of 2-Hp-β-CD); (b) the effect of the Rh6G amount on the relative fluorescence quenching degree of the composites (I0 is the original intensity of CdTe@2-Hp-β-CD, I is the intensity of the materials with the corresponding amount of Rh6G); (c) real-time fluorescence signals for CdTe@2-Hp-β-CD@Rh6G probes in response to coumarin; and (d) fluorescence emission spectra of CdTe@2-Hp-β-CD@Rh6G at different pH values (the inset is the change of fluorescent intensity at different pHs); (CdTe@2-Hp-β-CD@Rh6G: 0.5 mg mL−1 and coumarin: 25 μM). Error bars represent the standard deviations of three independent measurements.

The quenching test data of CdTe@2-Hp-β-CD by the Rh6G are shown in Fig. 4b. When Rh6G is added to CdTe@2-Hp-β-CD, the intensity of CdTe@2-Hp-β-CD is reduced. As the mass ratio of CdTe@2-Hp-β-CD to Rh6G increases to 1[thin space (1/6-em)]:[thin space (1/6-em)]1, the quenching efficiency remains unchanged. Therefore, the optimal mass ratio of CdTe@2-Hp-β-CD to Rh6G is speculatively 1[thin space (1/6-em)]:[thin space (1/6-em)]1, and excess Rh6G can be washed off by the purification. Therefore, the ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]1 was selected for the synthesis experiment.

To identify the best response time, the real-time fluorescence signals for the CdTe@2-Hp-β-CD@Rh6G probe in response to coumarin at different reaction times were surveyed (Fig. 4c). It shows that the quenching degree increases with the incremental reaction time up to 100 min. Hence, an optimal response time of 100 min was chosen for sensing experiments. The acidity for determining the concentration of coumarin was also investigated. A certain amount of coumarin was added to the CdTe@2-Hp-β-CD@Rh6G system, and the fluorescence intensity was measured. The pH value of the reaction system was set to be 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, and 13, respectively. At a pH value of 7, the fluorescence emission intensity is the highest, as shown in Fig. 4d. Therefore, the acidity of the reaction system was set to pH = 7.

Sensing process

To study the sensitivity of the nanoprobe, the effect of coumarin concentrations on the fluorescence intensity of CdTe@2-Hp-β-CD@Rh6G was also investigated (Fig. 5a). With increasing concentrations of coumarin, more and more coumarin can replace Rh6G, and then the FRET is impaired. The fluorescence intensity gradually increases with the addition of 0 to 25 μM coumarin. There is no obvious variation for the peak position of 554 nm, indicating that the QDs can remain monodisperse after being added with coumarin. It shows that 2-Hp-β-CD can act as a protective ligand to avoid the aggregation of the particles. Therefore, CdTe@2-Hp-β-CD@Rh6G can serve as a high-efficiency probe for coumarin sensing. To realize the quantitative detection of coumarin, Fig. 5b depicts the linear relation between the fluorescence recovery degree at 554 nm and the concentration of coumarin. The recovery degree increases linearly with the incremental concentration of coumarin in the range of 0–25 μM. (I–I0)/I0 = 0.0004 + 0.0305 Ccoumarin (R2 = 0.9986) is its linear equation with a limit of detection of 0.63 μM.46
image file: d4nj02455a-f5.tif
Fig. 5 (a) The fluorescence spectra of 0.5 mg mL−1 CdTe@2-Hp-β-CD@Rh6G added with 0–25 μM coumarin obtained using a 360 nm excited light source; (b) the linear relation between the fluorescence recovery degree at 554 nm and the concentration of coumarin (I0 is the original intensity and I is the intensity of the materials with corresponding concentrations of coumarin).

Interference study

In addition, the selectivity of the nanohybrids for coumarin-sensing was also studied. Interfering reagents are perhaps present in cosmetic samples, such as sodium benzoate, glycine, mannitol, glycerol, sodium alginate, carbamide, citric acid, and sodium citrate. Fig. 6(a) and (b) show that the intensity can become stronger distinctly after coumarin is appended, whereas no evident variations appear due to other reagents. Obviously, the designed sensor has excellent selectivity for coumarin sensing over other interfering substances. Therefore, the CdTe@2-Hp-β-CD@Rh6G nanoprobe can be used in the following direct sensing of coumarin in cosmetics.
image file: d4nj02455a-f6.tif
Fig. 6 (a) The fluorescence emission graphs of 0.5 mg mL−1 CdTe@2-Hp-β-CD@Rh6G added with 20 μM coumarin or 800 μM common substances (such as EDTA, sodium benzoate, glycine, mannitol, glycerol, sodium alginate, carbamide, citric acid, and sodium citrate) in cosmetic samples at 360 nm excitation. (b) The bar graph of the fluorescence recovery degree for the materials at 554 nm added with or without interfering reagents (I0 is the original intensity and I is the intensity with coumarin or other interfering substances).

Real sample testing

For the sake of evaluating the practicability of the proposed method, the FRET-based strategy was employed for the detection of coumarin in cosmetic samples. The results of coumarin detection by HPLC and the CdTe@2-Hp-β-CD@Rh6G nanoprobe are compared in Table 1. No coumarin was detected in blank cosmetic samples by HPLC. The detection results of the CdTe@2-Hp-β-CD@Rh6G nanoprobe were consistent with those of HPLC after the same concentrations of coumarin were added. The relative standard deviation (RSD) and recovery rates of coumarin detected by CdTe@2-Hp-β-CD@Rh6G in cosmetics were 1.89–3.60% and 96.0–105.6%, respectively. In addition, the T-test was employed to investigate whether there were some differences between the proposed probe and HPLC. The comparison revealed that there were no significant differences between the two methods at 95% confidence level, demonstrating that the proposed sensor is effective for detecting actual samples. These results indicate that the CdTe@2-Hp-β-CD@Rh6G nanoprobe can be used for the detection of coumarin in actual cosmetics.
Table 1 Spiked recovery results for coumarin sensing in cosmetic samples
Sample Original (μM) Coumarin added (μM) HPLC CdTe@2-Hp-β-CD@Rh6G T-testa
Coumarin detected (μM) Recovery (%) RSD (%, n = 5) Coumarin detected (μM) Recovery (%) RSD (%, n = 5)
a Values in parentheses are tabulated t values at P = 0.05. All the concentrations are the mean values of five determination (n = 5).
Sunblock 0 5.0 4.69 93.8 2.81 4.80 96.0 1.89 1.54 (2.31)
Mist spray 0 5.0 4.87 97.4 1.20 4.92 98.4 3.60 0.60 (2.31)
Emulsion 0 5.0 5.19 103.8 3.75 5.28 105.6 2.90 0.81 (2.31)

At the same time, Table 2 shows a comparison of the sensor prepared in this study with formerly reported sensors for the detection of coumarin in the last few years prepared using all kinds of methods. In terms of effectiveness and reliability, the constructed probe shows a comparable detection range and LOD for coumarin sensing with the other reported technologies.

Table 2 A comparison of the sensor prepared in this study with formerly reported sensors for the detection of coumarin
Probe LOD (μM) Linear range (μM) Literature
Fe3O4NPs-RB 0.11 3.4–171 Yang et al. (2022)47
Fluorescence spectroscopy 6.9 13.7–82.1 Polácek et al. (2015)48
GC 84.8 170–3420 Rahim et al. (2011)49
HPLC 1.37 3.4–684.3 Hrobonova et al. (2020)4
CdTe@2-Hp-β-CD@Rh6G 0.63 0–25 This work

Conclusions

In summary, a FRET-based nanohybrid to detect coumarin was successfully constructed. This nanoprobe used CdTe QDs as a reporter and Rh6G as a recognition unit. The fluorescence spectrum of QDs and the UV-vis spectrum of Rh6G have a good overlap. At the same time, 2-Hp-β-CD can act as a linker to reduce the distance between them. Therefore, the fluorescence intensity of CdTe QDs decreases, displaying an “off” state. In the presence of coumarin, Rh6G is replaced due to the greater association constant between coumarin and 2-Hp-β-CD than that of Rh6G and 2-Hp-β-CD, and then the emission intensity of QDs restores (“on” state). The CdTe@2-Hp-β-CD@Rh6G nanoprobe can act as a sensing system for practical coumarin detection in cosmetics. This general method could be expanded to other fluorescence sensing systems as a versatile platform for the detection of other analytes.

Author contributions

Ruoqian Xu: conceptualization, methodology, investigation, resources, and writing – original draft; Cai Shi: data curation and formal analysis; Xuemei Dong and Xuan Xiao: methodology, investigation, formal analysis, and validation; Yujie Ding – methodology, writing – review and editing, supervision, and funding acquisition.

Conflicts of interest

The authors declare that they have no known competing financial interest or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the scientific research planning project of the Department of Education in Anhui Province (2022AH050960), the overseas visiting and research project for Outstanding Young Talents of Colleges and Universities in Anhui province (gxgwfx2019043), the undergraduate scientific research project of Anhui Polytechnic University in 2022 (2022DZ25), the undergraduate innovation and entrepreneurship fund of the national grade (202310363066) and the joint training graduate research innovation fund project of Suzhou University (2023KYCX09).

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